Recently, research on an Orthogonal Frequency Division Multiple Access (OFDMA) scheme or a Single Carrier-Frequency Division Multiple access (SC-FDMA) scheme has been actively conducted for high-speed data transmission over a wireless channel in a mobile communication system.
The 3rd Generation Partnership Project (3GPP), an asynchronous cellular mobile communication standard organization, has conducted research on Long Term Evolution (LTE) based on a multiple access scheme for high-speed packet data transmission. In addition, research for efficiently providing a Voice over Internet Protocol (VoIP) service in an LTE system has progressed.
VoIP service technology refers to a communication technique for generating voice data provided by a voice codec as an IP/UDP/RTP (Internet Protocol/User Datagram Protocol/Real-time Transfer Protocol) packet and transmitting the generated packet. It is technology for providing a voice service, which has already been provided through a circuit switched network such as Public Switched Telephone Network (PSTN), also through an IP network.
Requirements for delay in the VoIP service should be met in order to provide the VoIP service, which is delay-sensitive, through a wireless packet access system or a wireless data packet communication network.
When Dynamic Scheduling (DS), a scheme for performing scheduling every time data is transmitted, is applied, resource allocation control information for notifying information on allocated resources is increasingly generated for scheduling the VoIP service in which data having a small size is frequently generated. Therefore, it becomes difficult to cope with the delay of a VoIP packet. The increase of the resource allocation control information signifies the decrease of resources for transmitting user data. As a result, it causes the reduction of overall system capacity. In this regard, the VoIP service, in which data having a small size is frequently generated, needs to reduce the amount of control information, which increases in proportion to the number of users, for securing system capacity for user data transmission.
As an attempt to reduce the amount of the control information, various schemes for semi-persistent resource allocation have been discussed. In the LTE system, a method for allocating resources by using a Semi-Persistent Scheduling (SPS) scheme is proposed for an efficient VoIP service.
In the SPS scheme, when the VoIP service begins, a base station transmits uplink resource allocation control information to a terminal by using Physical Downlink Control CHannel (PDCCH) Downlink Control Information format 0 (DCI0). Then, the base station and the terminal perform an operation for a VoIP service, which is generated thereafter, according to the transmitted control information. Resources, which have been allocated by transmitting the control information, are valid before the SPS is activated or deactivated/released.
In resource allocation through the SPS as described above, the base station may undergo a problem (i.e. a blind detection problem) such that it does not know whether transmission by the terminal is initial transmission or re-transmission. In order to solve the blind detection problem of the base station, a scheme is used for fixing a period of initial transmission so that the terminal may perform the initial transmission by each predetermined period. Namely, the period of the initial transmission is provided to the terminal through Radio Resource Control (RRC). The initial transmission by the terminal is performed by each transmission period (e.g. 20 ms).
For example, the LTE system allocates uplink resources of the terminal in the SPS scheme for the VoIP service. However, given resources are limited. Therefore, when the number of users who require a service becomes large, or when conditions of a wireless channel for transmitting a VoIP packet are not good, allocatable resources become insufficient.
Also, a VoIP packet generated from a voice codec has compression ratios of IP/UDP/RTP which are changed by a Packet Data Conversation Protocol (PDCP) layer according to call quality. Namely, when voice call quality is poor, the PDCP layer reduces a header compression ratio, and copes with call quality deterioration. The compression ratio reduction causes the generation of a VoIP packet having a large size. Then, the VoIP packet having a large size requires the allocation of more resources. As a result, allocatable resources become insufficient. Due to the insufficient resources, new packets whose transmission has been delayed are accumulated in a buffer of a Medium Access Control (MAC) unit of a transmitter. The packets accumulated as above cause continuous service delay. Moreover, they do not satisfy a requirement of the VoIP service for which a packet should be transmitted in a predetermined time.